Natural hybrid orbitals: Ab initio SCF and CI results for CO and NiCO

Abstract

The natural hybrid orbital (NHO) procedure of Foster and Weinhold is generalized to treat ab initio SCF or CI wavefunctions in minimal or extended basis sets. The first applications of the ab initio NHO analysis to a series of wavefunctions for carbon monoxide and the NiCO complex are presented. Numerical and graphical comparisons of NHOs for these systems exhibit the effects of basis-set augmentation and electron correlation on the details of the natural “Lewis diagrams”. Hybrid p-character and bond polarization are found to be persistent features of the NHO description, well represented even at the minimal basis level. Extension of the basis set allows the natural lone pairs to delocalize somewhat, while the principal effect of electron correlation is to shift occupancy somewhat into the π*CO orbitals. The effect of coordination of CO to a Ni atom, as conventional chemical intuition would suggest, is to polarize the carbon lone pair more toward the metal atom and the πCO bond more toward the oxygen.